1. Field of the Invention
The present invention relates to a display, and particularly, to a display such as an electroluminescence display with a plurality of emission layers laid one over another, to display gradational images.
2. Description of Related Art
Organic electroluminescence displays (OELDs) are drawing attention because they are very thin and are high in contrast, response speed, and viewing angles. Hereafter, the organic electroluminescence display is sometimes referred to as “organic EL display.” The organic EL display uses organic compounds that demonstrate an electroluminescence effect to convert electricity into light. The organic EL display frequently employs an active matrix driving method to improve energy efficiency and reduce energy consumption. Due to these and other advantages, the organic EL display is regarded as a promising next-generation display.
FIGS. 1A and 1B show layer structures employed for conventional organic EL displays. FIG. 1A is a monolayer structure for an organic EL display. On a glass substrate 40, an anode 41 is formed. The anode 41 is a transparent ITO (indium tin oxide) electrode to transmit light therethrough. The ITO electrode is widely used in, for example, liquid crystal displays.
On the anode 41, an emission layer 42 is formed. The emission layer 42 is made of organic luminescent compounds. There are low-molecular organic materials and high-molecular organic materials. For the emission layer 42, a proper material is selected according to the characteristics of the material and the usage and manufacturing method of the organic EL display. On the emission layer 42, a cathode 43 is formed. According to the related art shown in FIG. 1A, light produced in the emission layer 42 is emitted from the glass substrate 40, and therefore, the cathode 43 is a metal electrode made of, for example, aluminum.
A power source 44 applies a DC voltage between the anode 41 and the cathode 43, so that the anode 41 injects holes into the emission layer 42 and the cathode 43 injects electrons into the emission layer 42. The injected holes and electrons recombine in the emission layer 42 to form an excited state that is an unstable high-energy state. Just after that, the holes and electrons quickly return to a ground state that is a stable low-energy state. At this time, the emission layer 42 discharges energy as light.
FIG. 1B shows a five-layer structure for an organic EL display. The five-layer structure of FIG. 1B has a glass substrate 45 on which an anode 46, a hole injection layer 47, a hole transport layer 48, an emission layer 49, an electron transport layer 50, an electron injection layer 51, and a cathode 52 are sequentially formed.
To easily take holes from the anode 46, the related art of FIG. 1B employs the hole injection layer 47 and the hole transport layer 48. The layer 48 transports holes from the hole injection layer 47 to the emission layer 49. To easily take electrons from the cathode 52, the related art employs the electron injection layer 51 and the electron transport layer 50. The layer 50 efficiently transports electrons from the electron injection layer 51 to the emission layer 49.
A power source 53 applies a DC voltage between the anode 46 and the cathode 52 so that holes from the anode 46 are passed through the hole injection layer 47 and hole transport layer 48 into the emission layer 49. At the same time, electrons from the cathode 52 are passed through the electron injection layer 51 and electron transport layer 50 into the emission layer 49. The injected holes and electrons recombine in the emission layer 49 to from an excited state that is an unstable high-energy state. Just after that, the holes and electrons quickly return to a ground state that is a stable low-energy state. At this time, the emission layer 49 discharges energy as light.
With these layers of different roles, the organic EL display efficiently emits light.
Layer structures are not limited to those shown in FIGS. 1A and 1B. There are other structures involving two to four layers. Depending on the characteristics of an emission layer and electrodes, the hole injection layer, hole transport layer, electron injection layer, and electron transport layer may be optionally selected and combined. Also, various organic materials are selectable for the organic EL display. Some organic materials provide both the functions of hole injection layer and hole transport layer.
As explained above, the organic EL display has the anode 41 (46) and cathode 43 (52) that sandwich the organic emission layer 42 (49) that emits light.
FIG. 2 is a sectional view showing an example of a pixel structure in an organic EL display according to a related art. On a glass substrate 54, a silicon oxide film 55 and a thin film transistor (TFT) 59 are formed. The silicon oxide film 55 functions to prevent metal ions from migrating from the glass substrate 54 to an anode 56. The TFT 59 is used to turn on and off the pixel. On the silicon oxide film 55, the anode 56 is formed. On the anode 56, an organic emission layer 57 is formed. The organic emission layer 57 is a combination of an emission layer, a hole injection layer, a hole transport layer, an electron injection layer, and an electron transport layer.
On the organic emission layer 57, a cathode 58 is formed. The cathode 58 is made of metal such as aluminum. This related art employs a bottom emission structure in which light produced in the organic emission layer 57 is emitted from the glass substrate 54 having the TFT 59.
To improve light emission efficiency, a top emission structure that emits light from the cathode 58 is also possible. In this case, the cathode 58 is made of transparent ITO.
To display an image on a display, many pixel driving methods have been developed. One popular method used to display a gradational image on an organic EL display provides each pixel with an analog memory and a voltage-current converter and controls a drive current to an organic EL element of the pixel according to a voltage of the analog memory. FIG. 3 shows an example of a display panel for displaying gradational images with the use of such a conventional driving method.
In FIG. 3, a pixel 67 (to be explained later in detail) has an organic EL element and an active element and is driven by display panel drivers. In the display panel, a plurality of pixels 67 are arranged in a two-dimensional matrix.
The display panel has a horizontal driver 68 to drive the pixels 67 in a horizontal direction. A power source circuit 69 supplies a source voltage to all of the pixels 67. A vertical driver 70 drives the pixels 67 in a vertical direction. To drive, for example, a top display line, a gate driver 65 (shown in FIG. 4) in the vertical driver 70 supplies a voltage to the top display line to turn on the pixels 67 in the top display line. At this time, gate drivers 65 in the vertical driver 70 for display lines other than the top display line supply a voltage to turn off the pixels 67 in the display lines other than the top display line.
At the same time, the horizontal driver 68 outputs voltages corresponding to an image signal for one scan line (display line), so that capacitors 62 of the pixels 67 in the top display line may receive data voltages. As a result, in the top display line, terminals of each capacitor 62 receive the source voltage and data voltage, respectively, and the capacitor 62 sufficiently accumulates charge to maintain the potential difference between the terminal voltages. Namely, the display data is written into the capacitors 62 in the top display line and is kept therein. Thereafter, the vertical driver 70 is turned off, and the organic EL elements 60 of the pixels 67 in the top display line emit light according to the data stored in the capacitors 62, to thereby display an image on one scan line. Thereafter, the vertical driver 70 sequentially drives the pixels 67 line by line from the top to the bottom of the display panel. In synchronization with this, the horizontal driver 68 outputs image data line by line, to thereby scan all pixels 67 in the display panel.
FIG. 4 is a circuit diagram showing one of the pixels 67. The organic EL element 60 corresponds to the organic emission layer 57 of FIG. 2. Data that determines the brightness of light emitted from the organic EL element 60 is provided through a data input terminal 64. The gate driver 65 applies a voltage to the gate of a TFT 61 to turn on the TFT 61, and data from the terminal 64 is transferred through the source and drain of the TFT 61 to the capacitor 62 and the gate of a TFT 63. The voltage of the data controls a current supplied through the source and drain of the TFT 63 to the organic EL element 60.
Thereafter, the TFT 61 is turned off, and a potential difference between a power source 66 and the data input terminal 64 is stored in the capacitor 62. The organic EL element 60 is a current-driven emission device, and therefore, the brightness of light emitted from the organic EL element 60 is proportional to a current applied thereto. Namely, the brightness of light emitted from the organic EL element 60 is dependent on the potential of data supplied through the terminal 64. According to the data, the gradation of an image to be displayed on the display is determined.
FIG. 2 showed a conventional layer structure of an organic EL display and FIG. 4 showed a circuit for driving such a display to display gradational images. There are other driving methods. For example, a clamped inverter method conducts analog modulation on emission periods in each frame according to a pulse width modulation (PWM) signal. (Refer to, for example, “An Innovative Pixel-Driving Scheme for 64-Level Gray Scale Full-Color Active Matrix OLED Displays,” SID2002, 32.2.) This method compensates for threshold variations intrinsic to TFTs serving as active elements and reduces the number of TFTs to realize a simple circuit. This method employs no-emission periods to clearly display moving images.
There is a digital display driving method that turns on and off a switching transistor of each pixel, to control the ON/OFF state of an organic EL element of the pixel. An example of this method is disclosed in, for example, Japanese Unexamined Patent Application Publication No. Hei-10-214060. This disclosure divides an image into a plurality of subframes along a time axis and expresses a gradation level based on the total of weights of the subframes.
There is an area dividing method (for example, Japanese Unexamined Patent Application Publication No. Hei-11-073159) that divides a pixel into a plurality of sub-pixels in a screen of a display and expresses a gradation level based on the number of sub-pixels that emit light. There is another method (for example, Japanese Unexamined Patent Application Publication No. 2003-280593) employing sub-pixels. This method drives, in an analog mode, first sub-pixels to display a halftone image with multiple gradation levels, and at the same time, drives second sub-pixels to display a binary image with bright and dark levels.
The display driving method employing the circuit of FIG. 4 arranges the analog memory and voltage-current converter in each pixel, to express gradation. This method controls a current for driving an organic EL element according to a voltage of the analog memory. The characteristics of TFTs serving as active elements greatly vary from pixel to pixel, to vary a current passed to the organic emission layer. This results in varying the brightness of light from pixel to pixel, to cause brightness unevenness over the display and deteriorate the quality of displayed images.
An improvement of this method is the clamped inverter method disclosed in the above-mentioned “An Innovative Pixel-Driving Scheme for 64-Level Gray Scale Full-Color Active Matrix OLED Displays,” SID2002, 32.2. This method is effective to eliminate variations among TFTs of a conventional display. This method employs an analog emission period modulation technique that only turns on and off TFTs and controls an emission period with an analog pulse width (PWM) to express a gradation level. Accordingly, this method can reduce the influence of TFT variations. This method, however, is disadvantageous in the length of service life of a display because a current necessary for an intended brightness level is entirely passed to one emission layer to increase load on the emission layer. In addition, the method has a flickering problem because each gradation level is achieved with emission and no-emission periods.
The digital driving method employing subframes disclosed in the Japanese Unexamined Patent Application Publication No. Hei-10-214060 drives a display in a binary mode to turn on and off an organic EL element in each pixel, to thereby eliminate brightness variations. To display a gradation level, this method divides a frame period into a plurality of subframe periods, scans all pixels in every subframe period, writes binary display data having a gradational bit configuration into each pixel, and turns on each pixel for a predetermined time at a predetermined brightness level corresponding to the intended gradation level. This digital driving method using subframes may eliminate the influence of the TFT variations. However, it has an interference problem of causing, for example, pseudo contours on moving images.
The area dividing methods disclosed in the Japanese Unexamined Patent Application Publications No. Hei-11-073159 and No. 2003-280593 necessitate a plurality of sub-pixels for every pixel, and therefore, are not suitable for the trend of high-resolution panels in the future.